1. Field of the Invention
This disclosure is related to the field of polymer interlayers for multiple layer panels and multiple layer panels having at least one polymer interlayer sheet. Specifically, this disclosure is related to the field of polymer interlayers having improved optical properties, and more specifically, to polymer interlayers comprising a high refractive index plasticizer.
2. Description of Related Art
Multiple layer panels are generally panels comprised of two sheets of a substrate (such as, but not limited to, glass, polyester, polyacrylate, or polycarbonate) with one or more polymer interlayers sandwiched therebetween. The laminated multiple layer glass panels are commonly utilized in architectural window applications, in the windows of motor vehicles and airplanes, and in photovoltaic solar panels. The first two applications are commonly referred to as laminated safety glass. The main function of the interlayer in the laminated safety glass is to absorb energy resulting from impact or force applied to the glass, to keep the layers of glass bonded even when the force is applied and the glass is broken, and to prevent the glass from breaking up into sharp pieces. Additionally, the interlayer may also give the glass a much higher sound insulation rating, reduce UV and/or IR light transmission, and enhance the aesthetic appeal of the associated window. In regard to the photovoltaic applications, the main function of the interlayer is to encapsulate the photovoltaic solar panels which are used to generate and supply electricity in commercial and residential applications.
The interlayer may be a single layer, a combination of more than one single layer, a multilayer that has been coextruded, a combination of at least one single layer and at least one multilayer, or a combination of multilayer sheets.
In order to achieve the certain property and performance characteristics for the glass panel, it has become common practice to utilize multiple layer or multilayered interlayers. As used herein, the terms “multilayer” and “multiple layers” mean an interlayer having more than one layer, and multilayer and multiple layer may be used interchangeably. Multiple layer interlayers typically contain at least one soft layer and at least one stiff layer. Interlayers with one soft “core” layer sandwiched between two more rigid or stiff “skin” layers have been designed with sound insulation property for the glass panel. Interlayers having the reverse configuration, that is, with one stiff layer sandwiched between two more soft layers have been found to improve the impact performance of the glass panel. Examples of multiple layer interlayers also include the interlayers with at least one “clear” or non-colored layer and at least one colored layer. Other examples of multiple layer interlayers include interlayers with at least two layers with different colors. The colored layer typically contains pigments or dyes or some combination of pigments and dyes. The layers of the interlayer are generally produced by mixing a polymer resin such as poly(vinyl butyral) with one or more plasticizers and melt processing the mix into a sheet by any applicable process or method known to one of skill in the art, including, but not limited to, extrusion, with the layers being combined by processes such as co-extrusion and lamination. Other additional ingredients may optionally be added for various other purposes. After the interlayer sheet is formed, it is typically collected and rolled for transportation and storage and for later use in the multiple layer glass panel, as discussed below.
Contemplated polymer interlayers include, but are not limited to, polyvinyl acetals (PVA) (such as polyvinyl butyral (PVB)), polyurethane (PU), poly(ethylene-co-vinyl acetate) (EVA), polyvinylchloride (PVC), polyethylenes, polyolefins, ethylene acrylate ester copolymers, poly(ethylene-co-butyl acrylate), silicone elastomers, epoxy resins, and acid copolymers such as ethylene/carboxylic acid copolymers and its ionomers, derived from any of the foregoing possible thermoplastic resins. Multilayer laminates can include multiple layer glass panels and multilayer polymer films. In certain embodiments, the multiple polymer films in the multilayer laminates may be laminated together to provide a multilayer film or interlayer. In certain embodiments, these polymer films may have coatings, such as metal, silicone or other applicable coatings known to those of ordinary skill in the art. The individual polymer films which comprise the multilayer polymer films may be laminated together using an adhesive as known to those of ordinary skill in the art. The following offers a simplified description of the manner in which multiple layer glass panels are generally produced in combination with the interlayers. First, at least one polymer interlayer sheet (single or multilayer) is placed between two substrates and any excess interlayer is trimmed from the edges, creating an assembly. It is not uncommon for multiple polymer interlayer sheets or a polymer interlayer sheet with multiple layers (or a combination of both) to be placed within the two substrates creating a multiple layer glass panel with multiple polymer interlayers. Then, air is removed from the assembly by an applicable process or method known to one of skill in the art; e.g., through nip rollers, vacuum bag or another deairing mechanism. Additionally, the interlayer is partially press-bonded to the substrates by any method known to one of ordinary skill in the art. In a last step, in order to form a final unitary structure, this preliminary bonding is rendered more permanent by a high temperature and pressure lamination process, or any other method known to one of ordinary skill in the art such as, but not limited to, autoclaving.
One of the problems in the manufacture of multilayer laminate glass panels having multiple layer interlayers is the presence of mottle in the final unitary structure. The term “mottle” refers to an objectionable visual defect in the final unitary structure, namely the appearance of uneven spots, a form of optical distortion. Stated differently, mottle is a measure of the graininess or texture formed from the optical effect of reflecting non-uniform distorted interfaces of the inner polymer interlayer or polymer interlayers.
In multiple layer interlayers having at least one soft layer and at least one stiff layer, the mottle is caused by small scale surface variations at the interfaces between the layers wherein the individual layers (or the soft and stiff layers) have different refractive indices. When the polymer interlayer is produced, surface roughness is formed at the utmost surface of the polymer interlayer through melt fracture or embossing or both. The surface roughness enables and improves removal of air during laminating of polymer interlayers to produce multiple layer glass panels, and helps to prevent the blocking of the polymer interlayers during storage. Meanwhile, such surface roughness will also cause the development of small scale surface variation at the interfaces between layers of the multilayer interlayer.
The refractive index of a substance, such as an interlayer, is the measure of the speed of light through the substance with respect to the speed of light in vacuum. If there is a difference between the refractive index of the layers, the result will be that the surface variations are visible or even more visible due to diffraction of the light at the layer interfaces. Mottle is theoretically possible with any multiple layer interlayer, especially where there is a sufficiently large difference in the refractive indices between the layers and there is some degree of interfacial variation between the layers.
The presence of mottle in the final unitary structure of a multilayer laminate glass panel can be problematic because a certain degree of optical quality is necessary in many (if not most) of the end-use commercial applications of multilayer laminate glass panels (e.g., vehicular, aeronautical and architectural applications). Thus, the creation of multilayer laminate glass panels with commercially acceptable levels of mottle (that is, where the level of mottle is low) is paramount in the art of multiple layer glass panel manufacturing.
To ascertain the level of mottle in a laminate, the severity of the mottle is assessed and categorized by a side-to-side qualitative comparison of the shadowgraph projections for a test laminate with a set of standard laminate shadowgraphs representing a series or scale of mottle values ranging from 1 to 4, with 1 representing a standard of low mottle (i.e., a low number of disruptions) and 4 representing a standard of high mottle (i.e., a high number of disruptions), which is optically objectionable. Based upon a visual interpretation of which standard laminate shadowgraph picture the test shadowgraph projection best corresponds with, the test laminate is then placed into the mottle category of the corresponding standard laminate. Shadowgraph pictures can also be analyzed by digital image analysis tools to give digitalized results or mottle ratings.
Clarity of the multiple layer panel is another important optical quality. Clarity is determined by measuring the level of haze in the multiple layer panel, as further described below. The level of haze must be very low so that the multiple layer panel is clear. In addition to haze, there are other optical quality defects, such as visible optical defects in the interlayer, that cause light scattering and make the defect visible to the eye that may cause optical distortion in the glass panel as well. Both haze and other visible optical defects are caused by light scattering due to the blending or mixing of materials, such as different polymers or plasticizers, together, or the contamination from such different polymers or plasticizers where there is a sufficiently large difference in the refractive index between the different polymers or plasticizers, the matrix and the contaminants, or both.
Summarized, optical quality defects such as mottle, haze and other visible optical defects are common problems in the field of multiple layer glass panels, particularly those used in applications which require higher levels of optical or visual quality. It is now common to use a multilayer interlayer in order to provide high performance laminates. The use of multilayer interlayers, however, has very often resulted in having optical defect problems, such as mottle. Accordingly, there is a need in the art for the development of an interlayer, and particularly a multilayer interlayer, that resists or prevents the formation of mottle without a reduction in other optical, mechanical, and acoustic characteristics of a multilayer interlayer, and a need for the development of any polymer interlayers including monolithic and multilayer interlayers that are free from haze and other visible optical defects.